A liquid crystal display (LCD) is an indirect emission type display device which does not self-emit light and displays an image by controlling transmittance of light incident from an external light source. The LCD includes a separate light source, that is, a backlight unit (BLU) at one side of a liquid crystal panel and displays the image by controlling transmittance of light emitted from the BLU. The backlight unit generally includes a light source such as a fluorescent lamp and a light emitting device (LED), a light guide plate, and optical sheets such as a diffusion sheet and a prism sheet. FIG. 1 is an exploded perspective view of a general liquid crystal display.
Referring to FIG. 1, the liquid crystal display includes a backlight unit 10 and a liquid crystal panel 20. Generally, the backlight unit 10 is provided at the rear side of the liquid crystal panel 20 so as to irradiate the light to the liquid crystal panel 20, and includes a light source 11, a reflection plate 12, a light guide plate 13, a diffusion sheet 14, prism sheets 15 and 16, and a protective sheet 17.
The light guide plate 13 shifts a progressing direction so as to irradiate the light emitted from the light source 11 to the liquid crystal panel 20 side, and the reflection plate 12 is disposed at the rear side of the light guide plate 13 and reflects the light emitted to the rear side of the light guide plate 13 to the light guide plate 13 to minimize the loss of light. The diffusion sheet 14 serves to uniformly disperse the light incident from the light guide plate 13, the prism sheets 15 and 16 concentrate the light incident through an optical pattern formed on the surface thereof to emit the concentrated light to the liquid crystal panel 20, and the protective sheet 17 is provided on the prism sheets 15 and 16 to serve to protect the prism sheets 15 and 16.
The prism sheet is formed on a light transmitting base film with an optical pattern layer on which a triangular array-shaped optical pattern having a slope of 45° is generally formed in order to improve front-directional luminance. Such an optical pattern is formed in a mountain shape, and thus the upper portion of the mountain is easily broken or distorted to be damaged due to external small scratches. Accordingly, when the optical pattern is damaged, due to a difference in a path of the emitted light between a damaged portion and a normal portion, the luminance deteriorates and defects of the image in the liquid crystal display occur. Therefore, when the prism sheet is produced, there is a case where the entire surface of the produced prism sheet is not used according to a position even by the minute defect. This causes reduced productivity and presently, affects the rise in costs.
Recently, with activation of the use of personal portable terminals such as a smart phone which operates by directly touching the liquid crystal display, the terminals have been small-sized and reduced in weight so as to be easily carried, and as a result, the thickness of the liquid crystal panel tends to be gradually decreased. As a result, when unnecessary pressure is applied through the liquid crystal panel, cases where the optical pattern of the prism sheet is damaged will frequently occur. Particularly, in the case of using a plurality of prism sheets for improving the luminance, an optical pattern layer of the prism sheet disposed at the lower side as well as an optical pattern layer of the prism sheet disposed at the upper side are pressed and deformed, and thus there is a problem in that distortion in the image displayed on the liquid crystal panel occurs. Accordingly, development of an optical sheet including an optical structure surface which can flexibly cope with force applied from the outside is much needed.